diff --git a/lib/AST/GenericSignatureBuilder.cpp b/lib/AST/GenericSignatureBuilder.cpp
index 488a44b6ff8a4..bb58412049496 100644
--- a/lib/AST/GenericSignatureBuilder.cpp
+++ b/lib/AST/GenericSignatureBuilder.cpp
@@ -4361,6 +4361,8 @@ ConstraintResult GenericSignatureBuilder::addTypeRequirement(
       
       if (subjectType->is<DependentMemberType>()) {
         subjectType = resolveDependentMemberTypes(*this, subjectType);
+      } else {
+        subjectType = ErrorType::get(subjectType);
       }
 
       auto invalidConstraint = Constraint<Type>(
diff --git a/validation-test/compiler_crashers_2_fixed/rdar58941114.swift b/validation-test/compiler_crashers_2_fixed/rdar58941114.swift
new file mode 100644
index 0000000000000..50db463dda4dd
--- /dev/null
+++ b/validation-test/compiler_crashers_2_fixed/rdar58941114.swift
@@ -0,0 +1,6 @@
+// RUN: not %target-swift-frontend %s -typecheck
+
+class C {}
+protocol Foo {
+  associatedtype X where C: X
+}